Thanks for your reply...
I agree that may be my code is not the best practice, as i learn Qt by myself only with doc and tutorials... but the program was working perfectly when i delivered it to the researchers... In fact, is working perfectly if i compile it on windows... few hour ago i borrowed a windows machine, installed qt, compiled the same project and worked just fine... So i doubt is really a code problem, because it should appear in both Windows and Linux, as the code is exactly the same...
I'm starting to think that is a linux driver problem, so i will try in another linux when i get the chance...
If someone has ubuntu 15.04 and is kind enough to reproduce this problem and tell me i would be very happy!
If anybody else reproduce this problem in other linux machine i will start a bug report